Francisella tularensis is a highly infectious bacterium that poses a serious threat as an agent of bioterrorism. A live whole cell vaccine is currently available for at-risk populations, but it is associated with incomplete immunity and side-effects. Studies of humans and mice vaccinated with this preparation indicate that humoral and cell-mediated immune responses are required for complete protection in the infected host. Currently, little is known concerning the virulence factors associated with F. tularensis that contribute to its ability to cause lethal disease. However, previous studies have identified the capsule and LPS as principle determinants of its pathogenic potential. Our analysis of the recently released genome of F. tularensis Schu S4 has shown that it possesses a single polysaccharide biosynthetic locus responsible for expression of one surface polysaccharide. Based on these data, we hypothesize that the previously described capsule and O-antigen of this organism actually represents a single O-antigen/capsule that has the same repeating unit structure, but is expressed as a distinct large molecular-weight polymer and smaller molecular-weight Lipid A-linked polysaccharide. We predict that this structure has a central role in the pathogenesis of this organism and can be used as the basis for novel glycoconjugate vaccines that will elicit complete protection against experimental tularemia. To address the structural nature of this virulence factor and its role in the pathogenesis and immunity to F. tularensis, we propose to: 1) Characterize the structural and genetic nature of the O-antigen/capsule; 2) Determine the role of the O-antigen/capsule in virulence; 3) Determine the humoral and cell-mediated immune responses to the O-antigen/capsule and proteins of F. tularensis; and 4) Develop a conjugate vaccine for F. tularensis infections. These studies will employ a proteomics-based approach to identify new immunogenic proteins from F. tularensis that can be used as carriers in the development of novel acellular glycoconjugate vaccines. It is anticipated that these vaccines will activate both humoral and cell-mediated immune responses and elicit complete protection against tularemia. Glycoconjugate vaccines have been among the most effective biologics ever developed for the prevention of bacterial infections. It is expected that this approach can be applied successfully to the development of a vaccine that can ultimately be tested in clinical trials for the prevention of tularemia.